1. Field of the Invention
The present invention relates to an apparatus for controlling a salient-pole DC brushless motor by controlling armature currents of the motor based on a rotor angle thereof which is detected without using a position detection sensor.
2. Description of the Related Art
For energizing a DC brushless motor to produce a desired torque, it is necessary to apply voltages to the armatures in suitable phases corresponding to the electric angle of the rotor (hereinafter referred to as xe2x80x9crotor anglexe2x80x9d) which has magnetic poles. There have been proposed various processes of detecting a rotor angle without using a position detection sensor in order to reduce the cost of the DC brushless motor and a motor control apparatus therefor by way of dispensing with any position detection sensor for detecting the rotor angle.
The inventors of the present application have proposed a rotor angle detector for detecting a rotor angle of a DC brushless motor without using a position detection sensor in an earlier patent application (Japanese patent laid-open publication No. 2002-320398). The disclosed rotor angle detector operates as follows: When high-frequency voltages are added to drive voltages that are applied to three-phase armatures of a salient-pole DC brushless motor, the rotor angle detector uses a detected value of a current flowing through the first-phase armature, a detected value of a current flowing through the second-phase armature, and high-frequency components depending on the high-frequency voltages to calculate a sine reference value depending on the sine value of a twofold angle which is twice the rotor angle of the DC brushless motor and a cosine reference value depending on the cosine value of the twofold angle. Then, the rotor angle detector calculates the rotor angle of the DC brushless motor using the sine reference value and the cosine reference value which have been calculated.
A motor control apparatus for controlling the DC brushless motor performs a feedback control process on currents flowing through the armatures of the DC brushless motor to determine drive voltages to be applied to the armatures so that detected values of the armature currents (hereinafter referred to as xe2x80x9cdetected armature currentsxe2x80x9d) will be equalized to predetermined command currents. Because the high-frequency voltages are added to the drive voltages, high-frequency components are added to the detected armature currents.
The motor control apparatus has low-pass filters to remove such high-frequency components from the detected armature currents. However, if the high-frequency components are not sufficiently removed from the detected armature currents, then the differences between the detected armature currents and the command currents increase due to the high-frequency components added to the detected armature currents, tending to lower the ability of the DC brushless motor to cause its output torque to follow the command currents in the feedback control process.
It is therefore an object of the present invention to provide an apparatus for controlling a DC brushless motor to reduce adverse effects which armature currents have on a feedback control process when high-frequency voltages for detecting a rotor angle are added to drive voltages of the DC brushless motor.
According to the present invention, there is provided an apparatus for controlling a salient-pole DC brushless motor having armatures in three phases, comprising voltage applying means for applying drive voltages to the armatures, high-frequency adding means for adding high-frequency voltages to the drive voltages, first current detecting means for detecting a current flowing through an armature in a first phase of the armatures in the three phases, second current detecting means for detecting a current flowing through an armature in a second phase of the armatures in the three phases, reference value extracting means for extracting a sine reference value depending on the sine value of a twofold angle which is twice a rotor angle of the motor and a cosine reference value depending on the cosine value of the two-fold angle, using a first current value detected by the first current detecting means and a second current value detected by the second current detecting means when the high-frequency voltages are added to the drive voltages by the high-frequency adding means, and high-frequency components depending on the high-frequency voltages, rotor angle calculating means for calculating a rotor angle of the motor using the sine reference value and the cosine reference value, three-phase/dq converting means for handling the motor as an equivalent circuit having a q-axis armature disposed on an q-axis in the direction of magnetic fluxes from a rotor of the motor and a d-axis armature disposed on a d-axis which is perpendicular to the q-axis, and calculating a detected q-axis current flowing through the q-axis armature and a detected d-axis current flowing through the d-axis armature based on the rotor angle of the motor which is calculated by the rotor angle calculating means, the first current value, and the second current value, and current control means for determining the drive voltages so that a q-axis reference current produced by passing the detected q-axis current through a low-pass filter and a d-axis reference current produced by passing the detected d-axis current through a low-pass filter will be equalized to a predetermined q-axis command current and a predetermined d-axis command current, respectively.
According to the apparatus for controlling a DC brushless motor of the present invention, the high-frequency voltages are set so that the direction of a revolving magnetic field generated when the high-frequency voltages are applied to the armatures of the motor and the direction in which the motor is rotated by the drive voltages are opposite to each other.
With the above arrangement, as described in detail later on, since the high-frequency voltages are set so that the direction of a revolving magnetic field generated when the high-frequency voltages are applied to the armatures of the motor and the direction in which the motor is rotated by the drive voltages are opposite to each other, the frequencies of high-frequency currents added to the detected q-axis current and the detected d-axis current which are calculated by the three-phase/dq converting means are higher than if the high-frequency voltages are set so that the direction of a revolving magnetic field generated when the high-frequency voltages are applied to the armatures of the motor and the direction in which the motor is rotated by the drive voltages are the same as each other.
Consequently, the ability of the low-pass filters to attenuate the high-frequency currents added to the detected q-axis current and the detected d-axis current is increased, reducing the difference between the q-axis reference current and the q-axis command current and the difference between the d-axis reference current and the d-axis command current due to the high-frequency currents. Therefore, the ability of the motor to cause its output torque to follow the d-axis command current and the q-axis command current is increased.
According to the method of detecting a rotor angle of a DC brushless motor of the present invention, there is provided a method of detecting the rotor angle of the DC brushless motor controlled by an apparatus, said apparatus comprising voltage applying means for applying drive voltages to the armatures, first current detecting means for detecting a current flowing through an armature in a first phase of the armatures in the three phases, second current detecting means for detecting a current flowing through an armature in a second phase of the armatures in the three phases, three-phase/dq converting means for handling the motor as an equivalent circuit having a q-axis armature disposed on a q-axis in the direction of magnetic fluxes from a rotor of the motor and a d-axis armature disposed on a d-axis which is perpendicular to the q-axis, and calculating a detected q-axis current flowing through the q-axis armature and a detected d-axis current flowing through the d-axis armature based on the rotor angle of the motor which is calculated by the rotor angle calculating means, the first current value, and the second current value, and current control means for determining the drive voltages so that a q-axis reference current produced by passing the detected q-axis current through a low-pass filter and a d-axis reference current produced by passing the detected d-axis current through a low-pass filter will be equalized to a predetermined q-axis command current and a predetermined d-axis command current, respectively.
The method is characterized by comprising the step of adding the high-frequency voltages, which are set so that the direction of a revolving magnetic field generated when said high-frequency voltages are applied to the armatures of said motor and the direction in which said motor is rotated by said drive voltages are opposite to each other, to said drive voltage, calculating a sine reference value depending on the sine value of a twofold angle which is twice a rotor angle of said motor and a cosine reference value depending on the cosine value of the twofold angle, using a first current value detected by said first current detecting means and a second current value detected by said second current detecting means when said high-frequency voltages are added to said drive voltages, and high-frequency components depending on said high-frequency voltages: and a step of calculating a rotor angle of said motor using said sine reference value and said cosine reference value.
With the above arrangement, the ability of the low-pass filters to attenuate the high-frequency currents added to the detected q-axis current and the detected d-axis current is increased, reducing the difference between the q-axis reference current and the q-axis command current and the difference between the d-axis reference current and the d-axis command current due to the high-frequency currents. Therefore, the ability of the motor to cause its output torque to follow the d-axis command current and the q-axis command current is increased, and thus the rotor angle of the motor can be detected.
According to the present invention, furthermore, the reference value extracting means comprises means for extracting the sine reference value and the cosine reference value respectively according to the following equations (1), (2)                               V          ⁢                      xe2x80x83                    ⁢          s                =                              ∫            0                                          2                ⁢                                  xe2x80x83                                ⁢                π                            ω                                ⁢                                    {                                                cos                  ⁢                                      xe2x80x83                                    ⁢                  2                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                  t                  ⁢                                      xe2x80x83                                    ⁢                                                            cos                      ⁡                                              (                                                                              ω                            ⁢                                                          xe2x80x83                                                        ⁢                            t                                                    +                                                                                    2                              3                                                        ⁢                            π                                                                          )                                                              ·                    I                                    ⁢                                      xe2x80x83                                    ⁢                  u                                -                                  cos                  ⁢                                      xe2x80x83                                    ⁢                  2                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                  t                  ⁢                                      xe2x80x83                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                                      t                    ·                    I                                    ⁢                                      xe2x80x83                                    ⁢                  w                                            }                        ⁢                          ⅆ              t                                                          (        1        )                                          V          ⁢                      xe2x80x83                    ⁢          c                =                  ~                                    ∫              0                                                2                  ⁢                                      xe2x80x83                                    ⁢                  π                                ω                                      ⁢                                          {                                                      sin                    ⁢                                          xe2x80x83                                        ⁢                    2                    ⁢                                          xe2x80x83                                        ⁢                    ω                    ⁢                                          xe2x80x83                                        ⁢                    t                    ⁢                                          xe2x80x83                                        ⁢                                                                  cos                        ⁡                                                  (                                                                                    ω                              ⁢                                                              xe2x80x83                                                            ⁢                              t                                                        +                                                                                          2                                3                                                            ⁢                              π                                                                                )                                                                    ·                      I                                        ⁢                                          xe2x80x83                                        ⁢                    u                                    -                                      sin                    ⁢                                          xe2x80x83                                        ⁢                    2                    ⁢                                          xe2x80x83                                        ⁢                    ω                    ⁢                                          xe2x80x83                                        ⁢                    t                    ⁢                                          xe2x80x83                                        ⁢                    cos                    ⁢                                          xe2x80x83                                        ⁢                    ω                    ⁢                                          xe2x80x83                                        ⁢                                          t                      ·                      I                                        ⁢                                          xe2x80x83                                        ⁢                    w                                                  }                            ⁢                              ⅆ                t                                                                        (        2        )            
where Vs: the sine reference value, Vc: the cosine reference value, Iu: the first current value, Iw: the second current value, and a: the angular velocity of the high-frequency voltages.
The reference value extracting means can extract the sine reference value (Vs) and the cosine reference value (Vc) from the first current value (Iu), the second current value (Iw), and the angular velocity (X) of the high-frequency voltages according to the above equations (1), (2). The rotor angle calculating means can then calculate a rotor angle of the motor using the sine reference value (Vs) and the cosine reference value (Vc) thus calculated.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.